1. Field of the Invention
The present invention generally relates to a differential amplifier, and more specifically to a common mode controlled differential gain boosting amplifier where the common mode voltage includes a mirrored voltage.
2. Description of the Related Art
In analog applications, where accuracy is important (i.e. analog to digital converters), ultra-high gain op-amplifiers are often a necessity. Maintaining op-amplifier loop stability is challenging in an ultra-high gain and high accuracy amplifier. Balancing speed with stability creates an immediate trade-off in performance versus accuracy. Single stage folded cascode amplifiers are easy to stabilize but do not offer ultra-high gain. Differential gain boosting amplifiers can offer an order of magnitude increase in the gain of a single stage cascoded op-amplifier. In prior art, differential gain boosting is limited to cascoded devices.
FIG. 1 shows a conventional fully complementary folded cascode amplifier with differential gain boosting amplifiers. Vinp and Vinn represent input voltages for both an n-MOS differential pair 1 and a p-MOS differential pair 2. Vinp and Vinn are fully differential with respect to each other. A Vtailn transistor and a Vtailp transistor respectively complete the n-MOS 1 and p-MOS 2 differential pairs. Vsrcp and Vsrcn are both DC bias voltages connected respectively to gates of p-MOS transistors and n-MOS transistors creating a p-MOS current source 3 and an n-MOS current source 4. The gate voltages of the p-MOS 3 and n-MOS 4 current sources do not move.
Voutp and Voutn are output voltages of the circuit and are indicative of Vinp and Vinn with an increase in gain. Voutp and Voutf are each connected to a p-MOS cascode 5 and an n-MOS cascode 10. The n-MOS cascodes 10 and the p-MOS cascodes 5 are connected respectively to the n-MOS current source 4 and the p-MOS current source 3. The cascodes increases the gain on Voutp and Voutn.
Vbp and Vbn represent bias voltages that would traditionally connect directly to the cascode gates. Vbp, Vbn, Vsrcn, and Vsrcp are all generated from reference currents.
Common mode feedback amplifiers 6 and 8 and fully differential high gain amplifiers 7 and 9 are included in this circuit. The common mode feedback amplifier 6 sets the common mode voltage around which the amplifier 7 swings to Vbn. The common mode feedback amplifier 8 sets the common mode voltage around which amplifier 9 swings to Vbp. The gates of the cascodes 5, 10 are connected to the output of the amplifiers 7, 9 and the inputs of the common mode feedback amplifiers 6, 8.
Therefore, the common mode amplifiers 6 and 8 are used to control the common mode of the differential gain boosting amplifier 7 and 9 outputs. The positive reference input to the common mode amplifier 6, 8 is the traditional biasing device of a cascode 5, 10. The differential gain boosting output common mode voltage Voutp and Voutn can then be tuned using traditional cascode biasing. Single stage amplifier gain is about 8000 or 78 dB (typical PVT).
However, the aforementioned conventional amplifier still limits differential gain boosting to cascodes.